Solid oxide fuel cells (SOFC) have an electrolyte comprising an oxide ion conductor such as stabilized zirconia, for example. In the solid oxide fuel cells, a membrane electrode assembly (MEA) including an anode and a cathode that are disposed on each side of the electrolyte is sandwiched by separators (bipolar plates). Usually, the solid oxide fuel cell is used in the form of a fuel cell stack comprising a predetermined number of stacked MEAs and separators.
The fuel cell stack is operated at a high temperature of 800° C. or higher. When the fuel cell stack starts to operate, therefore, it is desirable to quickly heat the fuel cell stack to the desired temperature with a combustor. The combustor is usually positioned at an oxygen-containing gas discharge outlet of the fuel cell stack. For example, there is known a solid electrolyte fuel cell electric generator disclosed in Japanese Laid-Open Patent Publication No. 6-76849.
In the disclosed solid electrolyte fuel cell electric generator, as shown in FIG. 28 of the accompanying drawings, air and fuel are heated from a normal temperature to a predetermined temperature by respective heat exchangers 1, 2, and thereafter supplied to a fuel cell 3. In the fuel cell 3, the supplied air and fuel initiate a generating reaction. An exhaust gas discharged from the fuel cell 3 after being used in the generating reaction is introduced into an afterburner 4 which serves as a combustor.
The afterburner 4 burns the fuel remaining in the exhaust gas and produces a combustion gas, which is supplied as a heating medium for heating the air and the fuel to the heat exchangers 1, 2. At this time, air at room temperature is added from an air supply system 5 to the combustion gas to be supplied to the heat exchangers 1, 2, thereby lowering the temperature of the combustion gas.
According to the conventional solid electrolyte fuel cell electric generator, however, since the fuel remaining in the high-temperature exhaust gas from the fuel cell 3 is burned by the afterburner 4 to produce the higher-temperature combustion gas, conditions to be met for designing the afterburner 4, e.g., conditions for selecting heat-resistant materials of the afterburner 4, are highly strict. Especially, since the afterburner 4 is exposed to the exhaust gas at high temperature throughout the operation, the durability of the afterburner 4 is lowered.
Furthermore, when the fuel cell 3 starts to operate, the temperature of the gas discharged from the fuel cell 3 varies. In order to keep the temperature of the exhaust gas in a given temperature range, the afterburner 4 needs to be controlled according to a complex control process. Therefore, the stability of the fuel cell 3 at the time it starts to operate is liable to be lowered.